Optical particle sensors operate on the principle of measuring the amount of light scattered or intercepted when an intensive illuminating beam is projected through a flow steam containing suspended particles. Flow cells for containing this liquid flow have been developed for accurately containing the sample liquid within the center of a circularly laminar flow of a sheath fluid. These flow cells were designed for forming a nonturbulent, laminar flowing fluid that was then caused to surround a sample-containing fluid. These flow cells allowed the exact axial positioning of the flow of sample fluid such that it passed a scanning or observing device. The above-mentioned flow cells were utilized in a slit-scan photometry device described by Wheeless, Jr. et al, in U.S. Pat. No. 3,657,537 entitled, "COMPUTERIZED SLIT-SCAN CYTOFLUOROMETER FOR AUTOMATIC CELL RECOGNITION."
In the Wheeless patent, a cytofluorometer was used to give graphic fluorescent contours of a fluorochromed cell. The technique described by Wheeless allowed nuclear fluorescence to be distinguished from nonspecific cytoplasmic fluorescence that was often observed in particles such as squamous cells. The shape of the pulse was an indication of the ratio of the areas of the nucleus and cytoplasm, which is an informative index. These squamous cells are generally flat and of so-called "fried-egg shape."
The term "fried-egg shape" is used to describe squamous cells because such cells are generally circular in plan view and have a somewhat elevated nucleus which may or may not be centered. This general configuration resembles the shape of a fried egg.
The optical scanning device used in the above-described system responded to the maximum cross-sectional area of the particle perpendicular to the direction of the light beam of the scanning device. Due to the irregular shape and unpredictable orientation of the squamous cell, the cross-sectional area presented to the scanning device was found to vary widely, thereby precluding proper measuring results.